This invention relates to controlling a refrigeration unit used to chill the interior of a mobile cargo container and, in particular, a method for holding the supply air temperature delivered to a cargo container within extremely close limits.
Many control systems found on later model air conditioning units used to cool the interior of refrigerated cargo containers include a processor that is programed to adjust a control valve mounted in the compressor suction line of the air conditioning unit. The valve is adjustable between a fully open and fully closed position. The processor receives supply air temperature information and adjusts the valve setting based upon a preprogrammed schedule in response to the deviation of the sensed supply air temperature from a predetermined set point temperature.
The program used to control the position of the suction valve typically has three terms that are summed to arrive at a desired valve setting. The terms are all based upon the amount of deviation between the sensed supply air temperature and the desired set point temperature. The program not only looks at present conditions, but also at the history leading up to the present condition. The first term in the formulation is a proportional term relating to the present deviation (P), the second term involves an integral term based upon accumulated supply air temperature data (I), and the last term is a derivative term based on changes in supply air deviations (D). This formulation has come to be known in the industry as a PID control program because of the nature of the three terms involved.
Each of the three terms in the PID control formulation is multiplied by a control constant. The constants are selected to maintain the supply air temperature as close as reasonably practical to the set point temperature when the refrigeration unit is operating under steady state conditions. When the supply air temperature deviates some small amount from the set point temperature, the processor adjusts the suction control valve setting to bring the temperature back towards the desired set point. However, when the deviation between the supply air temperature and the set point temperature is relatively large, as for example when a cargo container door is left open, or during start up, the time for the system to near the set point temperature may be relatively long and the cargo stored in the container may be endangered.
By the same token, the PID program is unable to maintain continuous control over the system when the cooling load is small, as for example, when the ambient temperature is very low. When the unit is operating at or close to minimum capacity the suction valve is typically fully closed and no further control can be exercised over the system. By the time control is regained the supply air temperature can deviate from the set point temperature to a point where a temperature sensitive cargo may be endangered. By the time the system has a chance to recover, the cargo may be damaged.
The PID constants used in a typical program are selected to provide for a reasonable recovery time while still being able to maintain the supply air temperature close to a desired set point temperature. It is, however, highly desirous when transporting certain temperature sensitive produce to maintain the container temperature within extremely close tolerances, that is, within 0.25 degrees C. of the desired set point temperature. Present day PID control systems cannot deliver this type of close control.